This invention relates to nondestructive testing of bodies or objects by magnetic particle inspection procedure, for detecting of defects and metallurgical conditions, e.g. cracks and flaws, particularly in the surface of the body, and is particularly concerned with simple methods for facile and rapid magnetic particle inspection of bodies utilizing a novel magnetic recording medium which permits magnetic recording of images of defects oriented at any angle, or of defects parallel to each other, and which also permits obtaining magnetic indications of the depth of a crack or defect, employing a viewing device, preferably containing weakly magnetic crystals, or other suitable viewing means, in conjunction with such novel magnetic recording medium.
Conventional magnetic particle inspection methods for nondestructive testing of bodies employ solutions or powders of magnetic material with no magnetic retentivity, such as iron oxides, which are applied to the surface of the object and build up over cracks or defects contained in bodies of magnetic materials such as steel. In these methods the magnetic particles generally are directly deposited over the surface of the objects or parts, contaminating them. Such magnetic particles are used up in the process and they are generally not recoverable since they attach or adhere to the magnetized surface of the parts being inspected. Thus, the magnetic particles in these methods contaminate the surface of the objects being inspected. Further in these conventional methods for nondestructive flaw and crack detecting, the magnetic flux applied to the object must be induced at right angles to the suspected cracks and defects for satisfactory results. Further, in conventional methods when 2 or more cracks are present at different angles, it is necessary to reorient the magnetizing field for each of such cracks independently and in separate operations; otherwise, the magnetic indications of the crack previously recorded will be erased or obliterated by the last "shot" of magnetizing flux.
U.S. Pat. No. 3,862,047 discloses casting replicas of holes in a workpiece, such as a fastener recess, employing a liquid vulcanizable silicone rubber containing a hydrocarbon solvent or diluent, such as naphtha, and a catalyst, and ferromagnetic or iron oxide particles. This liquid magnetizable composition is placed in the hole or fastener recess to be inspected, a magnetic field is applied prior to setting up, or curing the mixture, to cause the magnetic particles to migrate to any cracks or flaws which are present, and following curing or hardening, the solid rubber replica is removed from the hole or recess for visual hardening. However, such solid rubber replica is tailored for that specific use or hole alone, and cannot be employed for inspecting other cracks or defects on or closely adjacent the surface of the part.
In British Pat. No. 1,237,864 there is disclosed a method for examining ferromagnetic or magnetizable materials employing a magnetic recording medium comprising a pad of deformable plastic or resilient material impregnated with magnetic recording material which may be magnetizable particles, and which, following magnetization, produces a visible record of the magnetization pattern of the body and an indication of the surface features of the body. However, the deformable or resilient magnetic recording medium of this patent is not of a composition, particularly with respect to the magnetic particles employed therein, which will provide indications of cracks or flaws by means of invisible magnetic indications in the magnetized recording medium which are made visible through the use of suitable means, e.g. a viewer, and such magnetic recording medium does not have the characteristics which permit obtaining indications of cracks or defects oriented at any angle to each other, or which can be applied to indicate the relative depth of a recorded defect.
Further, conventional magnetic particle inspection procedure of the prior art and as used in nondestructive testing throughout the industry is only capable of revealing indications of defects usually in one general direction which is preferentially at a 90.degree. angle to the lines of magnetic flux applied to the ferromagnetic part being inspected. If the magnetic flux is re-oriented to be parallel to the first formed indications the indications will be erased. This is an extremely undesirable characteristic of all conventional nondestructive magnetic particle inspection test processes used today. Further, the depth of a crack or defect located with conventional methods can only be verified by destructively cutting the part at a 90.degree. angle to the crack or defect. Also, many of the prior art processes in which the parts are directly contacted with magnetic particles are contaminating to the parts being inspected due to the abrasiveness of the magnetic particles (iron oxides) being used.